1. Field of the Invention
The invention relates to an extreme UV radiation source device with the function of wavelength selection of extreme UV radiation from radiation which is emitted from a plasma. The invention also relates to an extreme UV radiation exposure tool using this extreme UV radiation source device as a radiation source.
2. Description of Related Art
According to the miniaturization and increased integration of an integrated semiconductor circuit, an increase in resolution is required in a projection exposure tool for purposes of its manufacture. To meet this requirement, the wavelengths of the exposure light source are being increasingly shortened. As a semiconductor exposure light source of the next generation in succession to an excimer laser device, an extreme UV radiation source device (hereinafter also called an EUV radiation source device) is being developed which emits EUV radiation (extreme ultraviolet radiation) with wavelengths from 13 nm to 14 nm, especially with a wavelength of 13.5 nm.
Several schemes are known for producing EUV radiation in an EUV radiation source device. In one, by heating and excitation of an EUV radiating fuel, a high density and high temperature plasma is produced and EUV radiation is extracted from this plasma.
The EUV radiation source device adopting such a scheme based on the method of producing a high density and high temperature plasma is roughly divided into an EUV radiation source device of the LPP (Laser Produced Plasma) type and into an EUV radiation source device of the DPP (Discharge Produced Plasma) type (for example, reference is made to “Current situation and future prospect of research on an EUV (extreme UV) radiation source for lithography”, J. Plasma Fusion Res. March 2003, Vol. 79, No. 3, pp. 219-260).
In an EUV radiation source device of the LPP type, EUV radiation from a high density and high temperature plasma is used which is formed when targets, such as solids, liquid, gas and the like, are irradiated with a pulsed laser.
On the other hand, in an EUV radiation source device of the DPP type, EUV radiation from a high density and high temperature plasma which has been produced by power current driving is used. The discharge method in an EUV radiation source device of the DPP type, as described in “Current situation and future prospect of research on an EUV (extreme UV) radiation source for lithography”, J. Plasma Fusion Res. March 2003, Vol. 79, No. 3, pp. 219-260, is a Z pinch method, a capillary discharge method, a dense plasma focus method, a hollow cathode triggered Z pinch method and the like. The EUV radiation source of the DPP type, compared to the EUV radiation source of the LPP type, has the advantages of a small radiation source device and a small power consumption of the radiation source system. Practical use in the market is strongly expected.
Since material for EUV radiation has very low transparency, reducing projection using a transparent optical system cannot be achieved. For an exposure optical system in an extreme UV radiation exposure tool using the EUV radiation source device as the radiation source (hereinafter also called an EUV exposure tool), therefore, solely a reflection optical system including a mask is used. The mirror which is used in this reflection optical system and which reflects EUV radiation with a wavelength of 13.5 nm is, for example, a multilayer mirror of Mo (molybdenum) and Si (silicon)
In FIG. 14, the arrangement of one example of an EUV exposure tool using an EUV radiation source of the DPP type is shown. As shown in FIG. 14, the EUV exposure tool formed, for the most part, of an EUV radiation source device 10 and an exposure machine 20.
In the figure, the EUV radiation source device of the DPP type has a chamber I as the discharge vessel in which, for example, a first ring-like main discharge electrode (cathode) 3a and a second ring-like main discharge electrode (anode) 3b are arranged such that a ring-like insulating material 3c is clamped by them. The chamber 1 is comprised of a first vessel 1a on the side of the first main discharge electrode of electrically conductive material and a second vessel 1b on the side of the second main discharge electrode, likewise, of electrically conductive material. The first vessel 1a and the second vessel 1b are separated and insulated from one another by the above described insulating material 3c. 
The ring-like first main discharge electrode 3a, the ring-like second main discharge electrode 3b and the insulating material 3c are arranged such that their respective through openings are arranged essentially coaxially, and thus, form a continuous opening.
The chamber 1 is supplied with a raw material which contains an EUV radiation fuel from a raw material supply unit 11 which is connected to a raw material feed opening 2 on the side of the first vessel 1a of the chamber. The above described raw material is, for example, SnH4 gas, Xe gas, or the like. Furthermore, a gas evacuation unit 13 is connected to a gas outlet opening 7 which is located on a side of the second vessel 1b of the chamber 1 and is used to evacuate the chamber and to adjust the pressure within the chamber based on the measured value of a pressure monitor (not shown) which monitors the pressure within the chamber.
Furthermore, within the second vessel 1b of the chamber 1, there is an EUV focusing mirror 5. The EUV focusing mirror 5 has several mirrors which are formed, for example, in the shape of ellipsoids of rotation with different diameters or in the form of paraboloids of rotation with different diameters. These mirrors are arranged coaxially such that the center axes of rotation come to rest on one another so that the focal positions essentially agree with one another. These mirrors can advantageously reflect EUV radiation with an oblique angle of incidence of 0° to 25°.
If, in such a radiation source device of the DPP type, between the first and the second main discharge electrodes 3a 3b, pulsed power is supplied from a high voltage pulse generating part 12, on the surface of the insulating material, a creeping discharge is formed, by which essentially a short circuit state is formed between the first and the second main discharge electrodes 3a, 3b and a pulse-like large current flows. In this connection, a plasma 8 is formed in the essentially coaxial through openings which are formed by the ring-like first main discharge electrode 3a, the ring-like second main discharge electrode 3b and the insulating material 3c, or in the vicinity of the through openings. Afterwards, an area of high density and high temperature plasma is formed by Joulean heating by the pinch effect essentially in the middle area of the above described plasma 8. EUV radiation with a wavelength of 13.5 nm is radiated from this area of a high density and high temperature plasma.
The EUV radiation emitted from the area of a high density and high temperature plasma with a wavelength of 13.5 nm is focused by the above described EUV focusing mirror 5 and extracted from an EUV radiation exit part 6 located in a second vessel 1b to a subsequent stage. This EUV radiation exit part 6 is coupled to an EUV radiation incidence part 22 which is located in the exposure machine frame 21 of the exposure machine. This means that the EUV radiation which has been focused by the focusing mirror 5 is incident in the exposure machine via the EUV radiation exit part 6 and the EUV incidence part 22.
In a radiation source device of the DPP type, there can also be a pre-ionization means with which the raw material which has been supplied to the chamber is subjected to pre-ionization when a discharge is produced in the chamber and which contains the EUV radiation fuel. When EUV radiation is produced, the pressure within the chamber is adjusted, for example, to 1 Pa to 20 Pa. Under such a low pressure, the formation of a discharge depending on the electrode arrangement is made difficult. As a result, there are also cases in which the output of the EUV radiation becomes unstable. In order to produce a stable discharge in a situation in which a discharge forms with difficulty, it is desirable to carry out pre-ionization.
Furthermore, between the area of the high density and high temperature plasma (for the arrangement of the example shown in FIG. 14, the essentially coaxial through openings which are formed through the ring-like first main discharge electrode 3a, the ring-like second main discharge electrode 3b and the insulating material 3c or the vicinity of these through openings) and the EUV focusing mirror 5, there is a debris trap 4 which is used to capture debris and the like and for transmission of only EUV radiation. The debris trap 4, as is described in Japanese Patent Application Publication JP-A-2002-504746 (U.S. Pat. No. 6,359,969 B1), is formed of several plates which are located in the radial direction of the producing area for a high density and high temperature plasma. This debris trap captures debris, such as metallic powders, particles and the like, which is produced by sputtering of a metal (for example, of the discharge electrodes) which is in contact with the high density and high temperature plasma, by the above described plasma, and captures debris, debris as a result of a radiating fuel, such as Sn or the like, and others.
The EUV radiation source device of the DPP type shown in FIG. 14 also has a radiation source control element 14 which controls the high voltage pulse generating part 12, the raw material supply unit 11 and the gas evacuation unit 13 based on an EUV emission command or the like from an exposure machine control element 41.
On the other hand, all components of the exposure machine 20, such as the illumination optical system 24, the mask 25, the projection optical system 26, the workpiece 27, the workpiece support 28 and the like are mounted in a vacuum, since EUV radiation is absorbed by air. These components, as is shown, for example, in FIG. 14, are all located within the exposure machine frame 21. The inside of the exposure machine frame 21 is evacuated by a gas evacuation unit 31 which is connected to a gas outlet opening 29 located in the exposure machine frame 21, and achieves a vacuum state.
As was described above, the EUV radiation exit part 6 located in the EUV radiation source device 10 and the EUV radiation incidence part 22 which is located in the exposure machine frame 21 are coupled to one another. The inside of the chamber of the EUV radiation source device 10 and the inside of the exposure machine frame 21 have arrangements which can be differentially evacuated by the respectively arranged gas evacuation units 13, 31.
The illumination optical system 24 which is located in the exposure machine 20 adjusts the EUV radiation incident from the EUV radiation incidence part 22 and illuminates the mask 25 of the reflection type in which a circuit pattern is drawn. As was described above, for the optical system within the exposure machine, a reflection optical system including a mask 25 is used. The illumination optical system 24 is formed of at least one optical element of the reflection type, such as a reflector or the like.
The radiation reflected by the mask 25 of the reflection type is projected onto the workpiece 27, reduced by the projection optical system 26. When a photoresist is applied to the workpiece 27, the circuit pattern of the mask which has been subject to reduced projection is formed on the above described resist. For the above described projection optical system 26 as for the illumination optical system 24, an optical system of the reflection type is used and is formed of at least one optical element of the reflection type, such as a reflector and the like. The illumination optical system 24 and the projection optical system 26, as shown in FIG. 14, are used to facilitate the description and do not constitute a practical arrangement of the optical elements.
Generally, the area which has been projected onto the workpiece is a circular arc-shaped slit area. By synchronous scanning of the mask support on which the mask is placed, and of the workpiece support onto which the workpiece is placed, with a speed ratio which corresponds to a reducing sensitivity factor, a rectangular exposure area is implemented.
The exposure machine 20 which is shown in FIG. 14 has an exposure machine control element 41 which controls the evacuation unit 31 on the side of the exposure machine, a mask support drive control element 33 which subjects the mask support 23 to driving control, and a workpiece support driving control element 32 which subjects the workpiece support 28 to driving control.
It goes without saying that the above described exposure machine 20, like the conventional exposure tool, has a workpiece alignment device, transport systems for the mask and the workpiece and the like, although they are neither shown in the drawings nor described in the text.
FIG. 15 schematically shows the arrangement of an example of an EUV exposure tool using an EUV radiation source of the LPP type. The EUV exposure tool shown in FIG. 15 has an EUV radiation source device 10 and an exposure machine 20, like the device shown in FIG. 14. Since the exposure machine 20 as shown in FIG. 15 has the same arrangement as the one shown in FIG. 14, it will no longer be described here, but an EUV radiation source device of the LPP type is described below.
As is shown in FIG. 15, the EUV radiation source device of the LPP type has a chamber 1 as the discharge vessel, which is penetrated by a nozzle 11a to which a raw material supply unit 11 is connected. The sealing action within the chamber 1 is maintained. Neither gas leaks nor the like occur out of the area of the chamber 1 which is penetrated by the nozzle 11a. In this arrangement, a raw material which contains an EUV radiation fuel is supplied from the tip of the nozzle 11a of the chamber 1. The above described raw material which has been supplied via the nozzle 11a of the chamber 1 is, for example, Xe or SnH4 and is supplied to the chamber in a liquified state, a gaseous state or the like.
Furthermore, a gas evacuation unit 13 is used to control the pressure of the generating area for a high density and high temperature plasma based on the measured value of a pressure monitor (not shown) which monitors the pressure within the chamber 1, and to evacuate the chamber 1, is connected to a gas outlet opening 7 which is located in the chamber 1.
A laser device 15 which is, for example, a pulsed laser device with a repetition frequency of a few kHz, a YAG laser, a carbon dioxide gas laser device, an excimer laser device or the like is used for this purpose. The laser beam emitted from the laser device 15 is fed into the chamber 1 through a laser beam incidence window 16 which is located in the wall of the chamber 1 by being focused by a laser beam focusing means 15a, such as a convex lens or the like.
The raw material supplied to the nozzle 11a is irradiated with a laser beam which has been admitted by the laser beam incidence window 16. The raw material is fed in the direction to the focusing point of the laser beam which is focused by the laser beam focusing means 15a. The raw material which has been irradiated with the laser beam is heated or excited, by which a plasma, including the area of the high density and high temperature plasma, is formed, from which EUV radiation with a wavelength of 13.5 nm is emitted.
The emitted EUV radiation is reflected by an EUV focusing mirror 17 located in the chamber 1 and is extracted by an EUV radiation exit part 6 which is coupled to the EUV radiation incidence part 22 which is located in the exposure machine frame of the exposure machine 20. This means that the EUV radiation focused by the EUV focusing mirror 17 is incident via the EUV radiation exit part 6 and the EUV radiation incidence part 22 in the exposure machine 20. The above described EUV focusing mirror 17 is, for example, a spherical mirror.
Between the high density and high temperature plasma 8 and the EUV focusing mirror 17 is the above described debris trap 4 which is used to capture debris and the like and for transmission of solely EUV radiation. The debris trap 4 is formed, as is described in Japanese Patent Application Publication JP-A-2002-504746 (U.S. Pat. No. 6,359,969 B1) of several plates which are arranged in the radial direction of the producing area for a high density and high temperature plasma. This debris trap captures debris, such as metallic powders, particles or the like, and captures debris which is formed from the radiation fuel, such as Sn or the like, and the like.
Furthermore, the EUV radiation source device of the LPP type shown in FIG. 15 has a radiation source control element 14 which controls the laser device 15, the raw material supply unit 11 and the gas evacuation unit 13 based on an EUV emission command or the like from an exposure machine control element 41.
It has been found that the radiation emitted from an EUV radiation source device contains not only EUV radiation with a wavelength of 13.5 nm, which is necessary for exposure, but also contains radiation outside of the wavelength of 13.5 nm (hereinafter also called “radiation outside the band”). This means that, in a process (ionization or excitation process) in which a raw material which contains an EUV radiation fuel is heated by laser radiation, a Z pinch effect after formation of a discharge, or by similar methods, the density and the temperature of the formed plasma are increased and EUV radiation with a wavelength of 13.5 nm is emitted, different energy transitions take place in which radiation with different wavelengths is emitted.
As was described above, the exposure optical system (illumination optical system, mask, projection optical system) for an EUV exposure tool is, for example, a reflection optical system using a multilayer Mo/Si mirror. A multilayer Mo/Si mirror has reflection properties not only for EUV radiation with a wavelength of 13.5 nm, but reflection properties relative to UV radiation, visible radiation and IR radiation. Therefore, radiation which is outside of the band and which is in the wavelength range outside of EUV radiation with a wavelength of 13.5 nm reaches the workpiece in the reflection optical system.
If a photoresist has been applied to the workpiece, exposure also takes place by radiation in a wavelength band of 150 nm to 300 nm, because a photoresist for EUV radiation with a wavelength of 13.5 nm often has a general sensitivity relative to radiation in the wavelength band from 130 nm to 400 nm (UV radiation), especially radiation in the wavelength band from 150 nm to 300 nm.
The image resolution R for exposure is R=kλ/NA, where λ is the exposure wavelength, NA is the numerical aperture and k is a constant. The resolution for exposure by radiation in the wavelength band from 150 nm to 300 nm which has longer wavelengths than EUV radiation is therefore lower than the resolution in exposure by EUV radiation. Specifically, if there is radiation in the wavelength band from 130 nm to 400 nm (UV radiation), especially radiation in the wavelength band from 150 nm to 300 nm, exposure with the desired resolution is not possible. This means that radiation in a wavelength band from 150 nm to 300 nm causes a reduction of the image resolution during exposure.
On the other hand, if radiation in the wavelength band of at least 800 nm (IR radiation) reaches the workpiece through an exposure optical system using a reflection optical system and is absorbed, thermal deformation of the workpiece itself is caused. The exposure surface of the workpiece therefore assumes an unwanted shape. As a result, the exposure performance decreases.
As was described above, radiation which is outside the band causes a degradation of the exposure performance of an EUV exposure tool. Therefore, it is necessary to reduce this radiation which is incident in the exposure optical system and which is outside the band as much as possible.
As one of the wavelength selection means for extraction of EUV radiation with a wavelength of roughly 13.5 nm which is necessary for exposure from radiation which is emitted by an EUV radiation source device, it can be imagined that an optical filter be used which transmits only the required wavelength bands. For example, a Zr (zirconium) thin layer filter transmits only EUV radiation with wavelengths from 5 nm to 20 nm. The transmittance of the EUV radiation with a wavelength of 13.5 nm for a Zr filter with a thickness, for example, of 200 nm is, however, roughly 50%. The usable intensity of the EUV radiation with a wavelength of 13.5 nm is therefore reduced considerably.
To increase the exposure throughput, it is necessary to increase the intensity of the EUV radiation incident in the exposure optical system with a wavelength of 13.5 nm, and thus, the amount of exposure. As described, for example, in “Development of the base technology of an extreme UV radiation (EUV) exposure system,” Research Report of the Technology-Research Association “Technology development organization for extreme UV radiation exposure systems (EUVA)”, p. 150, published on May 30, 2003, it was computed by way of a sample that to expose 100 wafers per hour, radiation with a wavelength of 13.5 nm with at least 115 W must be allowed to be incident in the exposure optical system.
As was described above, when using a Zr filter as the wavelength selection means, the transmittance of the EUV radiation with a wavelength of 13.5 nm is roughly 50%. Extraction of EUV radiation with the intensity required for exposure from an EUV radiation source device is therefore accompanied by difficulties.
Furthermore, it can be imagined that a diffraction grating can be used as another wavelength selection means. In this connection, radiation from an EUV radiation source device is incident in a diffraction grating and is spectrally decomposed, by which only EUV radiation with a wavelength of 13.5 nm is extracted.
However, since a diffraction grating is an optical element with which spectral decomposition takes place using angular dispersion of the wavelength, in the case of using radiation from an EUV radiation source device with low directivity, in contrast to laser radiation, it is necessary on the radiation incidence side of the diffraction grating to use a slit in order to limit the angle of incidence of the radiation emitted by the EUV radiation source device into the diffraction grating. Furthermore, if the reflectance of the reflection surface of the diffraction grating with respect to the EUV radiation and the diffraction efficiency of the diffraction grating with respect to the EUV radiation are considered, the intensity of the extracted EUV radiation decreases significantly, even if only EUV radiation with a suitable wavelength is selected and extracted using the diffraction grating from the radiation which is emitted by the EUV radiation source device.
That is, with consideration of the intensity of the EUV radiation which is necessary for exposure, use of a diffraction grating as a wavelength selection element is virtually impossible in practice.
As was described above, extraction of the EUV radiation with a wavelength of 13.5 nm with the intensity required for exposure by using a conventional wavelength selection means and by eliminating the radiation which is outside the band and which has adverse effects on exposure is extremely difficult.